Solar cell performance often benefits from the ability to form a band gap gradient in the absorber material. Efficiencies typically improve as the band gap increases towards the “back” of the absorber (away from the junction), and/or there is a slight bandgap increase near the junction. High-efficiency Cu(In,Ga)Se2 devices, for example, use a multi-stage co-evaporation process, in part to achieve this desired compositional gradient.
The Cu2ZnSn(S,Se)4 system would similarly benefit from the ability to form a graded-band gap absorber. The ratio of sulfur to selenium determines the band gap of CZTSSe. However, it is very difficult to form a S/Se gradient due to the fact that these species readily inter-mix at elevated temperatures.
An additional method of changing the band gap is to replace Cu, Zn, or Sn atoms with iso-valent species having different atomic/ionic radii. For example, it has been shown that introducing Ge as a replacement for a Sn atom increases the band gap. See, for example, Shu et al., “Cu2Zn(Sn,Ge)Se4 and Cu2Zn(Sn,Si)Se4 alloys as photovoltaic materials: Structural and electronic properties,” Physical Review B 87, 115208-1-6 (March 2013). This bulk effect has been demonstrated with nanocrystalline inks, however a band gap gradient has not been demonstrated. Cu2ZnSiS4 is also known to have a very wide band gap (i.e., about 3 electron-volts (eV)), therefore replacement of Sn with Si is expected to increase the band gap of CZTS(e).
Thus, improved techniques for achieving band gap grading in solar absorber materials would be desirable.